The present invention relates to a control stage for the drive motor of a recirculating pump unit of an antilock braking system (ABS). More particularly, the invention relates to a control stage in an ABS, operating on the recirculating principle, according to which brake fluid which is drained out during a pressure reduction phase at a wheel brake subjected to automatic control via a brake-pressure control valve, which is controlled by a pressure reduction signal of the electronic ABS control unit into its pressure reduction position, and a return line into a low-pressure accumulator, is pumped back by the recirculating pump or pumps into the main brake line of the respective brake circuit. As a result, a quantity of brake fluid corresponding to the drained quantity is recirculated into the brake booster, the pump drive being activated for at least the duration of the pressure reduction phase, and the drive motor being configured as an electric motor whose speed is proportional to the operating voltage.
The ABS, which is standard on the majority of Mercedes-Benz passenger cars, operates on the recirculation principle. According to that principle, brake fluid is drained out, during a pressure reduction phase, from a wheel brake subjected to automatic control and passes via a brake-pressure control valve, which is controlled by a pressure reduction signal of the electronic ABS control unit, into its pressure reduction position. The drained brake fluid then passes via a return line of the brake circuit to which the wheel brake subjected to automatic control belongs, into a low-pressure accumulator, from which the drained brake fluid is then pumped back by the recirculating pump assigned to the brake circuit in question into the main brake line of the brake circuit. As a consequence, brake fluid is also recirculated out of the main brake line into the brake booster outlet pressure space assigned to the brake circuit, as a rule of a tandem master cylinder. As a result, the brake pedal is pushed back in the direction of its basic position, and the driver notices this as a return indication of the ABS activation.
The recirculating pumps, each assigned individually to the two brake circuits, i.e. a front-axle brake circuit and a rear-axle brake circuit, and usually configured as free-piston pumps, have a common eccentric drive, which is driven by an electric motor. In the case of the standard ABS, this drive motor is driven at full power, i.e. at is full operating voltage and maximum speed, from the beginning of a pressure reduction phase introducing an antilock control cycle, so that the recirculating pumps are also operated at their maximum delivery from the beginning of the pressure reduction phase.
Consequently, when the ABS responds, even though only relatively slight brake pressure reductions, in absolute terms, at the vehicle wheels subjected to automatic control are necessary, a disproportionately vigorous, shock-like reaction on the brake pedal occurs, and this reaction is felt by drivers experiencing normal comfort to be troublesome at the least, and often even alarming.
This disadvantage can be avoided if the ABS is designed in such a way that it operates in the range of low amounts of necessary pressure reductions on the principle of pressure modulation by expansion of a modulation chamber and only in the range of higher amounts of necessary pressure reductions on a modified recirculation principle, in which the pressure modulator also assumes the function of a recirculating pump, so that a return indication of the activation of the ABS, noticeable at the brake pedal, takes place only in very much rarer cases and also less "violently", but realizing an ABS in such a way involves a considerably constructional outlay with regard to the design of the hydraulic unit and is very much more expensive than the hydraulic unit of the conventional ABS.
Another known antilock braking system, in which the problem of the pedal reaction when there is a response of the automatic control is less serious than in the case of an antilock braking system operating on the recirculation principle, is shown in DE 37 31 603 Al. This system operates on the principle of brake pressure reduction by draining brake fluid out from the wheel brake cylinder of one or more of the wheel brakes subjected to automatic control into the reservoir, and controls brake pressure restoring phases by at least one electrically driven pump which pumps brake fluid out of the reservoir into the wheel brake. During an activation of the automatic control, a quantity of brake fluid is pumped, also by the pump, into the master cylinder of the brake system such that its piston or pistons and, with it or them, respectively, the brake pedal assume a specified position provided for automatic control and corresponding to an intermediate position between their possible external positions. This setting of the specified position is necessary in order to exclude the possibility under extreme control conditions of a no-load controlling of the master cylinder, associated with which would be a failure of the brake system at the end of such a controlling operation.
In order to ensure as pleasant as possible a pedal feeling during the setting of the specified position of the master cylinder pistons and of a brake pedal provided for automatic control, and in order for operator convenience not to be adversely affected, it is also possible, as shown in DE 38 18 260 Al, for the speed of the pump to be controlled appropriately according to requirements on the basis of a "volume model". According to this model, the pump comes in at high speed if a considerable quantity of brake fluid has to be pumped into the master cylinder until the specified position of the brake pedal is reached, and at a lower speed if actual position and specified position of the brake pedal are only slightly different.
A significant disadvantage of such a known antilock braking system operating in control mode with a opening of the brake circuits to the reservoir is, however, as also know in combination with such a system shown in DE 38 13 172 Al, the considerable technical expenditure required for the functional monitoring of the pump. This monitoring is necessary for reasons of safety. Although it is possible to make use of sensors which are also employed for controlling the pump speed, such as wheel speed sensors, piston position sensors and/or pressure sensors, in the case of such an ABS at least the outlet valves assigned to the wheel brakes which can be subjected to automatic control must also be able to be checked for their serviceability This requires an additional expenditure in order to achieve an overall functional reliability comparable with the functional reliability of a brake system equipped with an antilock braking system operating on the recirculation principle, i.e. constantly with closed brake circuits
An object of the present invention is, therefore, to improve the standard ABS or a functionally equivalent ABS, with low outlays such that the return indication characteristic of a response of the ABS noticeable at the brake pedal is lessened considerably in its intensity while nevertheless remaining adequately perceptible.
This object has been achieved by a control stage according to the present invention in which, from the onset of a pressure reduction signal, the control stage generates an operating voltage for the drive motor which rises with increasing duration of this signal and, after a predetermined rising time, can reach a maximum value correlated with maximum speed of the motor. The voltage then is held at this value at least until the pressure reduction signal dies out, or otherwise rises up to that value which is reached within the pressure-reduction signal duration. The control stage controls a reduction, either constant or stepwise, of the operating voltage of the motor, commencing upon the dropping of the pressure reduction signal, until the standstill of the motor, that the time integral of the operating voltage over the reduction of the operating voltage corresponds at least to the time integral over the rising time of the operating voltage.
The type of actuation of the drive motor of the recirculating pumps provide according to the present invention has the effect that, in cases in which the brake pressure has to be lowered only by a relatively small amount and consequently pressure reduction signals of relatively short duration are generated, the speed of the drive motor of the recirculating pump is kept low. Consequently, the recirculation of the brake fluid into the brake line or lines takes place correspondingly slowly and the pedal reaction is accordingly also gentle. The return-indication movements of the brake pedal proceed as it were in a smoothed "ramp shaped" manner and not in an abruptly step-shaped manner which produces a much less disturbing pedal feeling when there is a response from the automatic control.
The control stage according to the present invention is also less expensive to implement since only additional electronic components which can be readily integrated into the circuitry of the electronic control unit of the ABS are required. Therefore, the control stage adds only a little, if at all, to the overall price of the ABS, which appears to be highly justified by the gain in comfort.
Due to the condition that the time integral of the operating voltage over its reduction phase is to be at least equal to the time integral of the recirculating-pump drive motor over its rising phase, it is also adequately ensured that the buffer/accumulators, which can receive brake fluid in pressure reduction phases of antilock control, is or are substantially pumped empty at the end of a pressure reduction phase and therefore able to receive inflowing brake fluid once again.
At least one digital/analog converter is provided for generating the operating voltage fed to the drive motor of the recirculating pumps which convert the output counter reading of a forward/backward counter into a voltage proportional thereto. During the duration of the pressure reduction signal, this forward/backward counter is fed at its plus counting input upward-counting pulses generated at a fixed clock rate. After dying-out of the pressure reduction signal, the minus counting input of the counter is fed continuously generated backward-counting pulses, which are suppressed for the duration of the pressure reduction signal. The repetition frequency of the backward-counting pulses is, at most, equal to that of the upward-counting pulses. This provides a way of realizing the basic configuration of the control stage according to the invention.
Digital-electronics can be integrated in a simple way into the electronic control unit of the ABS in accordance with the present invention. At least one AND logic element is provided for generating the upward-counting pulses, which element receives at one of two inputs the pressure reduction signal and at the other input high-frequency output pulses of a clock pulse generator. At least one AND logic element is provided for generating the backward-counting pulses, which element has at least one negated input, at which it receives the output signal of the respective AND element, as well as a non-negated input, at which it receives the output pulses of the clock pulse generator.
Other features of the present invention provide a control stage which is appropriately configured for a vehicle with front-axle/rear-axle brake circuit division and separate brake pressure control at the front-wheel brakes and joint brake-pressure control at the rear-wheel brakes.
The rise of the operating voltage and the reduction of the operating voltage can take place time-linearly, and the duration of the voltage reduction can be greater that the duration of the rise of the operating voltage of the drive motor to provide the basic measure by which additional certainly can be achieved that low-pressure accumulators provided for receiving brake fluid drained off in pressure reduction phases of antilock control are completely pumped empty at the end of a pressure reduction phase and consequently are receptive again with their full capacity for a subsequent reduction phase.
This measure can be realized in a simple way by the frequency of the backward-counting pulses fed to the minus counting input of the forward-backward counter being lower than that of the upward counting pulses and, in combination with the above-mentioned digital-electronics, of realizing the control stage according to the invention, in which the minus counting input of the forward/backward counter is preceded by a dividing stage.
With alternative features, including in combination with those features mentioned above in this respect, it can be ensured that buffer accumulators provided for receiving brake fluid are completely pumped empty after the pressure reduction phases have elapsed, it also being possible to utilize these possibilities. For instance, the signal, with is available as a direct-voltage signal for at least the duration of the pressure reduction phases and is fed to an input of the AND element, respectively, emitting the upward-counting pulses, is generated as a signal which commences with the pressure reduction signal but is drop-delayed with respect to the latter. Furthermore, at the beginning of braking, a supply voltage signal bringing about a brief activation of the recirculating pump drive motor is generated.